1. Field of the Invention
The invention relates to the field of earth drilling equipment, and more particularly, to an earth drilling apparatus attached at an end of a vehicle-mounted boom, especially to permit substantially vertical boring through hard earth, rock, or rock-like material, at a distance from the vehicle.
2. Prior Art
The electric utility industry relies heavily on overhead power lines to distribute electrical power to its various customers. High voltage power lines emanating from a generating station are strung between transmission towers. The high voltage power is stepped down to a lower voltage via transformers at an electrical substation, on utility poles, or elsewhere. Power lines carrying lower voltage power are strung between smaller utility poles and lead eventually to individual residences and businesses. Utility poles may support power lines, telephone lines, cable TV lines, etc., and similar poles may be used as pilings or footings for structures.
The utility poles typically are simply placed in the earth. The poles have a portion of their length, typically 10 percent plus 2 feet, embedded in the earth, which typically is filled or packed around the pole. The pole protrudes upwardly to provide a location for fixing cables and the like, or for attachment of structural members. Holes must be drilled in the earth to accept the embedded portion whenever a new or replacement utility pole is to be installed, and pole installation and maintenance are frequent and routine jobs for utility companies and the like. The standard pole hole must be in the range of 6 to 10 feet deep to accommodate a standard utility pole up to 17 inches in diameter at its widest part, and typically anywhere from 20 to 50 feet high.
The utility poles are commonly located at regularly spaced locations along the side of a roadway. The terrain along the side of the roadway may be flat or sloped, smooth or jagged. The earth itself may include sand, dirt, clay, broken or solid rock, or any combination of these. Often, the side of the roadway terminates in a steeply ascending or descending rock-faced cliff. It is advisable to support the utility lines at a regular spacing to avoid undue tension or an unduly low arc in the lines, and to avoid installing an unnecessary number of poles. Therefore, the utility industry has a need to drill pole holes in the earth regardless of the character and condition of the earth at the respective location.
Earth drilling bits and drives adapted for all types of material are known. The utility industry generally employs auger type drills for producing holes in loose material. The auger apparatus is mounted on a vehicle and typically includes a hydraulic motor and a helical auger bit which is rotated by hydraulic fluid pressure generated by a hydraulic pump on the vehicle. The auger apparatus may be mounted on a boom extending from the vehicle, and the boom is generally rotatable over a limited span to either side of the vehicle such that the auger can be positioned for drilling a hole anywhere within a range determined by maneuvering the vehicle and moving the boom to position the auger at a distance from the vehicle. An auger bit is not suitable for drilling through rock, broken rock or rocky soil, as the rock will quickly wear out the auger bit. Further, the auger apparatus is not suitable for drilling in steeply sloped earth because the apparatus is not supported near the auger bit, other than by the earth itself. Thus, the auger bit tends to slide downhill whenever hole drilling in steeply sloped earth is attempted.
Rock drill bits are also known. These bits may include, for example, a drilling face having one or more hardened cutting elements embedded in the face and protruding toward the rock to be cut. A percussion tool, typically operated by compressed air, is generally attached behind the rock drill bit for delivering repetitive blows through the rock drill bit to the rock. The percussion tool and the drill bit can be attached at the bottom of a drill string, all or part of which is also rotatable about an axis by a drill head attached to the top of the drill string. Rock drill bits are commonly used by water well drillers in conjunction with vehicle mounted drilling equipment. Water wells may reach a depth of several hundred feet or more, and the well drilling vehicles include a relatively tall mast so that the drill head can be raised to a height above the ground sufficient to enable long lengths of drill pipe to be inserted or removed laterally into the drill string beneath the drill head.
Water well drilling equipment is not suitable for drilling utility pole holes, which require a quick setup and only a short hole depth. Since the water wells extend much deeper than the utility pole holes, water well drilling equipment is necessarily large and expensive and is setup relatively permanently as compared to utility pole drilling needs. The tall mast may interfere with trees along the side of the roadway. Of even greater concern, in the case of hole drilling for replacement utility poles, is potentially disastrous contact between the mast and existing power lines. Further, the tall mast cannot be used as a practical matter to conveniently place a drilling apparatus over a roadway guardrail to drill a hole on the remote side thereof.
Rock drills also have been mounted on cranes, which likewise have large booms. U.S. Pat. No. 4,102,094 to MacKinnon discloses a drill mast attached at an upper end to a large boom. An adjustable brace extends between the boom and a lower part of the mast to stabilize the mast and maintain correct alignment of the mast above a hole being drilled. The mast disclosed by MacKinnon is complex and bulky and does not overcome the practical disadvantages of such apparatus with respect to the job of drilling regularly spaced utility pole holes, such as excess mast height, high cost and inconvenience.
Heretofore, attempts to employ a rock drill on a truck mounted boom of the type normally used for auger drills have not been very successful. The booms have free play in their joints and tend to flex under the load of rock drilling. As a result, the drill bit tends to walk across a surface instead of digging in, and after the hole is started the bit is easily deflected from drilling in a straight line. This results in jamming of the bit in the hole, broken bits, increased drilling time, improperly aligned holes, etc. Consequently, the standard method of drilling utility pole holes in rock is manual; that is, a man with a jackhammer breaks up the rock and the rock is removed with a shovel. This method is obviously time consuming, and a single hole may take a day or longer to drill to the required depth. There is thus a need in the utility industry for a drilling apparatus which will overcome these problems.